Manufacture of fine-grained material for use in medical devices

ABSTRACT

Medical devices are manufactured from fine grained materials, processed from of a variety of metals and alloys, such as stainless steel, cobalt-chromium and nickel-titanium alloys. A fine grained metal or alloy is formed from a specimen rapidly heated to its recrystallization temperature, and then subjected to high temperature, multi-axial deformation, for example, by heavy cross-forging or swaging. The deformed specimen may be cooled and reheated to a second recrystallization temperature. The metal or alloy in the specimen is then allowed to recrystallize, such that the grain size is controlled by quenching the specimen to room temperature. A desired medical device is then configured from the fine grained material. Decreasing the average grain size of a substrate material and increasing the number of grains across a thickness of a strut or similar component of the medical device increases the strength of the device and imparts other beneficial properties into the device.

BACKGROUND OF THE INVENTION

This invention relates to medical devices, and more particularly tomethods of manufacturing medical devices using fine grained metals andalloys.

A focus of recent development work in the treatment of heart disease hasbeen directed to endoprosthetic devices referred to as stents. Stentsare generally tubular shaped devices which function to maintain patencyof a segment of a blood vessel or other body lumen such as a coronaryartery. They also are suitable for use to support and hold back adissected arterial lining that can occlude the fluid passageway. Atpresent, there are numerous commercial stents being marketed throughoutthe world. Intraluminal stents implanted via percutaneous methods havebecome a standard adjunct to balloon angioplasty in the treatment ofatherosclerotic disease. Stents prevent acute vessel recoil and improvethe long term outcome by controlling negative remodeling and pinningvessel dissections. Amongst their many properties, stents must haveadequate mechanical strength, flexibility, minimal recoil, and occupythe least amount of arterial surface area possible while not havinglarge regions of unsupported area.

One method and system developed for delivering stents to desiredlocations within the patient's body lumen involves crimping a stentabout an expandable member, such as a balloon on the distal end of acatheter, advancing the catheter through the patient's vascular systemuntil the stent is in the desired location within a blood vessel, andthen inflating the expandable member on the catheter to expand the stentwithin the blood vessel. The expandable member is then deflated and thecatheter withdrawn, leaving the expanded stent within the blood vessel,holding open the passageway thereof.

Stents are typically formed from biocompatible metals and alloys, suchas stainless steel, nickel-titanium, platinum-iridium alloys,cobalt-chromium alloys and tantalum. Such stents provide sufficient hoopstrength to perform the scaffolding function. Furthermore, stents shouldhave minimal wall thicknesses in order to minimize blood flow blockage.However, stents can sometimes cause complications, including thrombosisand neointimal hyperplasia, such as by inducement of smooth muscle cellproliferation at the site of implantation of the stent. Starting stockfor manufacturing stents is frequently in the form of stainless steeltubing.

The structural properties of the material used for implantable medicaldevices can improve with a decrease in the grain size of the substratematerial. It has been observed that stents cut from fully annealed 316Lstainless steel tubing having less than seven grains across a strutthickness can display micro cracks in the high strain regions of thestent. Such cracks are suggestive of heavy slip band formation, withsubsequent decohesion along the slip planes. Reduction of the grain sizein the substrate material, such as stainless steel, will reduce oreliminate the occurrence of such cracks and/or heavy slip band formationin the finished medical device.

The grain size of a finished stainless steel or similar metal tubedepends on numerous factors, including the length of time the materialis heated above a temperature that allows significant grain growth. Fora metallic tube, if the grain size is larger than desired, the tube maybe swaged to introduce heavy dislocation densities, then heat treated torecrystallize the material into finer grains. Alternatively, differentmaterial forms may be taken through a drawing or other working and heattreat processes to recrystallize the tubing. The type and amount ofworking allowed depends on the material, e.g., ceramics may require ahigh temperature working step while metals and composites may beworkable at room temperature. Grain-size strengthening is where there isan increase in strength of a material due to a decrease in the grainsize. The larger grain-boundary area more effectively blocks dislocationmovement. The outer diameter of the tube usually requires a machiningstep of some sort to smooth the surface after the swaging process, andthe same may be true before the tubing can be properly drawn.

Commercially available 316L stainless steel tubing contains averagegrain sizes ranging from approximately 0.0025 inch (sixty four microns),ASTM grain size 5 to around 0.00088 inch (twenty two microns), ASTMgrain size 8. These grain sizes result in anywhere from two to fivegrains across the tube thickness, and the stent subsequentlymanufactured from the tubing, depending on the tube and stent strutthicknesses. Part of the limitation in achieving a finer grain size inthis material arises from the number of draws and anneals the tubingmust go through to achieve its final size. The potential for reducingthe grain size exists by reducing the required number of heat-processingsteps by reducing the starting size of the raw product that is thenprocessed down into the tubing.

Lowering the grain size and increasing the number of grains across thestrut thickness of a stent allows the grains within the stent to actmore as a continuum and less as a step function. The ideal result ofprocessing the material to a smaller grain size would result in anaverage grain size of between approximately one and ten microns, with asubsequent average number of grains across the strut thickness aboutseven or greater. Likewise, other medical devices will benefit from areduction in grain size such as guide wires, ring markers, defibrillatorlead tips, delivery system devices such as catheters, and the like.

What has been needed, and heretofore unavailable, in the art of medicaldevice design is fine grained metals and alloys that have uniform andpredictable properties and that contain grain sizes on the order of oneto ten microns. The present invention satisfies these and other needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention is directed to themanufacture and use of fine grained (less than twenty microns indiameter) metals and alloys for use in medical devices. Medical devicesconfigured from fine grained material have many uses, including, but notlimited to, incorporation into stents, embolic protection devices, graftattachment systems, guide wires, pacing leads for cardiac pacemakers,defibrillator lead tips, ring markers, catheters, delivery systems,anastomosis clips and other tube or wire implants. The present inventionfor manufacture and use of fine grained materials may be applied to theprocessing of a variety of metals and alloys, such as stainless steeland nitinol. Decreasing the average grain size of a substrate materialand increasing the number of grains across a thickness of a strut orsimilar component of the medical device may increase the strength andductility of the device or impart other beneficial properties into thedevice.

The present invention includes methods of manufacturing fine grainedmaterials for use in medical devices. Similarly, the present inventionincludes medical devices made from such fine grained materials. Thenovelty of the fine grained manufacturing process includes subjecting aspecimen of a metal or metal alloy to multi-axial deformation (forexample, by heavy cross-forging or swaging) at elevated temperatureswithin the recrystallization regime of the specimen. The specimen isthen cooled to about room temperature to halt the recrystallizationprocess, so as to achieve the desired grain size in the specimen.

A variety of manufacturing methods may be employed to manufacturemedical devices of the present invention from a fine grained material.Such medical devices may be formed from a tube made from a fine grainedmaterial by laser cutting the pattern of the device in the tube. Themedical device also may be formed by laser cutting a flat fine grainedmetal (alloy) sheet in the pattern of the device, rolling the sheet intoa tubular shape and then providing a longitudinal weld to form thedevice, such as a stent. In addition, such a device may be formed from awire or elongated fiber constructed from fine grained material. Stentsand other implantable devices formed from such fine grained materialsmay be used with conventional over the wire or rapid exchange deliverysystems, and deployed into a patient's vasculature in a conventionalmanner.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph of annealed 316L stainless steel.

FIG. 2 is a transmission electron micrograph of fine grain stainlesssteel embodying features of the present invention.

FIGS. 3A and 3B are transmission electron micrographs of fine grainstainless steel embodying features of the present invention.

FIG. 4 is an elevational view, partially in section, of a fine grainstent embodying features of the invention, wherein the stent is mountedon a rapid-exchange delivery catheter and a fine grain guide wire.

FIG. 5 is an elevational view, partially in section, of a fine grainstent embodying features of the invention, wherein the stent is expandedwithin an artery, so that the stent apposes an arterial wall.

FIG. 6 is an elevational view, partially in section, of an expanded finegrain stent embodying features of the invention, wherein the stent isimplanted within an artery after withdrawal of a delivery catheter.

FIG. 7 is a side view of a fine grain stent embodying features of theinvention, wherein the stent is in an unexpanded state.

FIG. 8 is a side view of the fine grain stent of FIG. 7 in an expandedcondition, depicting cylindrical rings connected by undulating links.

FIG. 9 is a side view of a fine grain stent embodying features of theinvention, depicting cylindrical rings at the end of the stent having athicker cross section than the rings at the center of the stent.

FIG. 10 is a plan view of a flattened fine grain stent embodyingfeatures of the invention, illustrating a combination of undulatinglinks and straight links.

FIG. 11 is a perspective view of a fine grain stent embodying featuresof the invention, depicting cylindrical rings connected by straightlinks.

FIG. 12 depicts a longitudinal plan view of an embodiment of an expandedembolic protection device, including fine grain expandable struts of thepresent the invention.

FIG. 13 depicts a longitudinal plan view of the embolic protectiondevice of FIG. 12, wherein the device is collapsed for delivery into acorporal lumen.

FIG. 14 depicts a perspective view of a graft assembly, including aplurality of fine grain attachment systems of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings for purposes of illustration, the presentinvention is directed to the manufacture and use of fine grained metalsand alloys for forming material such as tube stock, a piece of tubing, awire and the like for use in intravascular and other medical devices.The novelty of the fine grained manufacturing process of the presentinvention includes subjecting a specimen of a metal or metal alloy tomulti axial deformation (for example, by heavy cross forging or swaging)at elevated temperatures within the recrystallization regime of thespecimen. The specimen is then cooled to about room temperature to haltthe recrystallization process, so as to achieve the desired grain sizein the specimen.

The fine grained material manufactured by the process of the presentinvention has many uses, including, but not limited to, incorporationinto medical devices, such as a stents, embolic protection devices,graft attachment systems, guide wires, pacing leads for cardiacpacemakers, defibrillator lead tips, ring markers, catheters, deliverysystems, anastomosis clips and other tube or wire implants. The presentinvention for manufacture of fine grained hypotube and wire may beapplied to many material types, including, but not limited to,processing of 316L stainless steel, cobalt-chromium alloys,nickel-titanium alloys, platinum-iridium alloys, titanium and titaniumbased alloys, tantalum and tantalum based alloys, and other suitablealloy systems. While virtually any medical device that is implanted orused in the body will benefit from the present invention, the inventionas applied to stents is described herein as only an example and is notmeant to be limiting.

One embodiment of the present invention is the grain refinement of 316L,an austenitic stainless steel that is widely used in medical devices(FIG. 1). One motivation for such grain refinement is to produce amaterial that will have uniform and predictable properties in sizes thatmay be less than one hundred microns in various dimensions, which can beon the order of the grain size in annealed material. Current 316Lstainless steel hypotube used in manufacturing medical device productscan have average grain sizes ranging from approximately 0.0025 inches(sixty four microns), ASTM grain size 5, to around 0.00088 inches(twenty two microns), ASTM grain size 8. These grain sizes result in anaverage of two to five grains across the wall thickness of the hypotube,and result in a similar number of grains across the width or thicknessof a stent strut manufactured from the hypotube. Lowering the grain sizeand increasing the number of grains across the strut thickness allowsthe grains within the stent to act more as a continuum and less as astep function, thereby providing a better distribution of the stresseswithin the grains to other grains, so as to increase the strength andductility of the fine grained material.

The ideal result of processing 316L stainless steel for use in a medicaldevice to a smaller grain size would result in an average grain size ofless than twenty microns (“fine grained”). With respect to medicaldevices that incorporate 316L stainless steel struts, shafts, links andsimilar elongated elements (such as stents and graft attachment systems)having a thickness of about one hundred microns, such elements shouldhave an average of about seven or more grains across the thickness ofthe element (preferably five to twenty grains across). Similarly, forsuch devices having a strut thickness of about fifty microns (such asportions of embolic protection devices), such elements should have anaverage of about three or more grains across the thickness of theelement (preferably two to fifteen grains across). Likewise, for medicaldevices incorporating 316L stainless steel wires or wire like elements(such as guide wires, pacing leads and defribillator leads) having adiameter of about three hundred and fifty microns, the wire should havean average of about thirty or more grains across the thickness of theelement (preferably twenty to sixty grains across). Similar calculationsmay be readily made by one of ordinary skill in the art for varyingdimensions of the device, and for different metals and metal alloys.

There are four generic ways to produce a fine grained material:recrystallization, martensite reversion, powder consolidation and metalinjection molding. The present invention is directed torecrystallization of metals and alloys, for example, 316L stainlesssteel. If a material is severely deformed and then heated to asufficiently high temperature the microstructural damage caused by thedeformation is relieved by the nucleation of new, nominally defect-freegrains which grow together to consume the deformed material. Thisprocess is known as “recrystallization,” which occurs above atemperature, the “recrystallization temperature,” whose value depends onthe extent of prior deformation. In the example case of 316L stainlesssteel, that temperature is several hundred degrees centigrade (° C.).

The grain boundaries and local damage sites in the deformed materialprovide the preferential sites for nucleation of the recrystallizedgrains. If the material is severely deformed the density of such sitesis very high and recrystallization begins with a dense shower of nuclei.The growing nuclei impinge on one another very quickly during growth toform a fine grained product. The fine grain size can be preserved byquenching the material to low temperature to prevent further graingrowth.

There are two basic ways to use recrystallization to obtain fine grainsize, as is well known to one of ordinary skill in the art. The simplestis to impose a severe deformation at high temperature, above therecrystallization temperature, so that recrystallization occurs almostimmediately after deformation, then quench the material rapidly to roomtemperature. The most obviously attractive method of high temperaturedeformation is hot forging at high strain rate, though other methods,such as rapid rolling or swaging, can also be used. In current practice,high-temperature, high-rate forging has been used to achieve one microngrain size in ferritic steel. Prior to the present invention, theapplicability of such recrystallization to 316L austenitic stainlesssteel for use in medical devices had not yet been demonstrated.

The second method to use recrystallization to obtain fine grain size isto deform the steel while it is cold, for example, by cold drawing,swaging or hydrostatic extrusion, and recrystallize by subsequentheating. This method offers a possible advantage in that it may be morecompatible with near-net shape forming operations to manufacturesuitable tubes. However, it is likely to be more difficult to use forgrain refinement, since it requires close control over the heating cycleused to accomplish the recrystallization.

There are two generic ways to form material into small diameter, thinwalled tubes (hypotube) and wire. The material may be created with finegrains then formed after the fact, or the material may be formed into afine grain using near-net shape processes where the hypotube or wireformation is part of the operation that refines the grain size. Drawinga material created with fine grains into seamless hypotube may bechallenging, since grain refinement increases the strength of thematerial via the Hall-Petch effect. However, two methods of forming amaterial created with fine grains into seamless hypotube are currentlyavailable.

In the first method, the material may be grain refined then formed intoa small diameter rod or wire by shape rolling, swaging or hydrostaticextrusion. The wire may then be gun drilled and machined into a tube ofsuitable dimensions. Alternatively, the grain refined material may bedrawn into a relatively large diameter tube, filled with a substancethat is easily deformed, such as aluminum or copper, then wire drawn orhydrostatically extruded into a small diameter wire. Afterwards thefiller material may be melted, etched or dissolved out and the tubingfinished to final specifications.

The second method of forming processes includes near-net shapetechniques that form the tube either before or concomitantly with grainrefinement. When recrystallization is used to accomplish the grainrefinement, then the deformation that drives recrystallization can beused to form a wire or hypotube through wire drawing, tube drawing,rolling or extrusion techniques. If the deformation is per formed athigh temperatures, recrystallization will accompany it. If thedeformation is performed at low temperature, recrystallization can beaccomplished by heating the material after the deformation operation. Ineither case, there is the potential of recrystallization producing somegeometric distortion which may then be corrected using finishingoperations to achieve the specified dimensions and shape.

The extent of recrystallization on hot deformation is the result of acompetition between two processes. The first is plastic deformation,which deforms the grains and introduces the excess energy that is thedriving force for recrystallization. To achieve uniformrecrystallization, the deformation must not only be extensive, but alsoreasonably homogeneous, so that all grains are deformed, and stableagainst recovery for a long enough time to nucleate recrystallization,so that the driving force is preserved. The second process istemperature based. As the temperature becomes very high, plasticdeformation becomes ineffective. The microstructure softens with theconsequence that deformation becomes inhomogeneous, leaving relativelyundeformed islands in the micro structure, and recovery becomes rapidand competes with recrystallization as a mechanism for relieving thestrain. A grain size of about three to six microns is close to theoptimal size for use in medical devices, since the grains are smallenough to ensure polygranular behavior in thin sections while keepingthe strength low enough for reasonable formability.

By way of example, the recrystallization method of the present inventionmay be used on annealed or cold worked specimens of starting material,such as 316L stainless steel. The annealed and cold-worked specimensyield similar microstructures in the as forged condition, since the hotdeformation overwhelms any prior deformation. Moreover, the hardness ofthe specimens is not changed dramatically by the forging process of thepresent invention. The hardness of the annealed specimens may increaseslightly, while that of the cold worked specimens may decrease slightly.In both cases, the as-forged hardness for 316L stainless steel uponcompletion of the recrystallization of the present invention is expectedto be about 29 Rc.

The recrystallization process of the present invention includes hightemperature multi axial deformation (for example, by heavy forging orcross-forging), which may be in the range of seventy five to ninety fivepercent net deformation of the target specimen. To aid in the forgingprocess, a suitable forging machine may be used, such as the oneconstructed at the National Research Institute for Metals in Tsukuba,Japan (NRIM). The Japanese NRIM forging machine contains two opposed TiC(titanium carbide) anvils, both of which move into the specimen in theform of a 15×15×100 mm metal bar. Other sized bars or metal billets maybe used, depending on the forging apparatus used. The NRIM machineforges at a set strain rate, which is achieved by controlling the finaldisplacement of the anvils. The two anvils are moved equally toward oneanother, so that the neutral axis remains at the centerline of thespecimen, and so that there is no macroscopic bending of the specimen.The specimen may be first forged to a selected displacement, and thenrotated ninety degrees and forged to a second set displacement. Forexample, the specimen may be first forged to a displacement of fiftypercent, rotated and then forged to a displacement of fifty percent,yielding a net deformation of eighty percent.

A suitable multi-axial forging or swaging apparatus should have thecapability of either static or continuous multi axial forging orswaging. Such a machine should have the capability to deform a targetspecimen in more than one orthogonal direction. For example, themulti-axial deformation may be performed in thirty, sixty or ninetydegree increments. The range of deformation may surround the specimen(360°), or other suitable increments, such as two ninety degreedeformations. An example of a static multi axial forging machine existsat NRIM.

In addition, a suitable multi axial forging or swaging machine shouldalso include the capability to heat the specimen to a variety oftemperatures at various heating rates, or should be able to accept aspecimen pre-heated at the appropriate rate from another machine priorto insertion in the forging or swaging machine. Those of ordinary skillin the art of metallurgy will recognize that the rate of heating thespecimen to the recrystallization temperature should avoid recovery (forexample, at about ten ° C. per second or less). Likewise, it should berecognized that while faster heating rates may be employed, care shouldbe taken to avoid distorting the shape and configuration of thespecimen.

In addition, such an apparatus should be capable of drawing a vacuumaround the specimen or maintaining an atmosphere of inert gas or othergas compatible with the specimen, so as to prevent oxidation of themetal during the forging or swaging process. Further, the apparatus or aclosely situated other apparatus should allow the capability to quicklyreduce the temperature of the specimen by any suitable means to controland halt the growth of the metal grain size, for example, by aircooling, or by quenching with water, oil or liquid nitrogen. The machineshould have the capability of either static or continuous multi-axialforging or swaging.

Fine-grained stainless steel may be formed using the recrystallizationprocess of the present invention. In accordance with the inventiveprocess, a 316L stainless steel cold-worked or annealed specimen isplaced in a forging machine or similar apparatus. Air is then evacuatedfrom the forging machine, and an inert gas or other gas compatible withthe specimen to prevent oxidation may be introduced. The stainless steelspecimen is then heated to a desired temperature in the range of about800 to 1100° C. The temperature of the specimen in the machine is thenstabilized.

In one embodiment of the recrystallization process of the presentinvention, the 316L stainless steel specimen is forged or heavilydeformed in multiple passes at the stabilized temperature to a high netdeformation, for example, eighty percent. The specimen is then held atthe stabilized temperature for one to ten minutes to allowrecrystallization, and then quenched to about room temperature to fixthe grain size. The expected resulting grain size of the fine grainedmaterial from this process is one to ten microns (less than elevenmicrons).

In another embodiment of the recrystallization process of the presentinvention, the 316L stainless steel specimen is heavily forged at thestabilized temperature, for example, using fifty percent plus fiftypercent cross forging to yield eighty percent net deformation. Afterforging, the specimen is then cooled to about room temperature in water,or by other suitable means. The stainless steel specimen is then heatedat a moderate rate (for purposes of illustration, at about ten ° C. persecond) to avoid too much recovery before reaching the recrystallizationtemperature in the range of 800 to 900° C. The specimen is held at thattemperature for one to ten minutes to control the grain size, and thenthe specimen is quenched to about room temperature to haltrecrystallization. The expected resulting grain size of the fine grainedmaterial from this process is one to ten microns (less than elevenmicrons).

Further embodiments of the process of the present invention may be usedto form fine grained materials using other metals and alloys, by varyingthe recrystallization temperature and time to achieve the desired grainsize. This elevated temperature, multi axial deformation process appliesto a wide range of metals and alloys, which may employ heat treatmentschedules commonly utilized for annealing or recrystallization purposes.Such heat treatment schedules are well known to those of ordinary skillin the art. Example alloys for which the fine grained manufacturingmethod of the present invention may be applied include, but are notlimited to:

-   -   1. L 605 (ASTM F90 and AMS 5759), a Co—Cr—W—Ni alloy also        available as STELLITE 25 (Deloro Stellite Company, Inc., South        Bend, Ind., U.S.A.) and HAYNES 25 (Haynes International Inc.,        Kokomo, Ind., U.S.A.), which should be heated to a        recrystallization temperature ranging between 1120 and 1230° C.,        and must have rapid cooling (e.g., air) in order to avoid        precipitation of undesirable phases;    -   2. ELGILOY (ASTM F1058), a Co—Cr—Mo—Ni alloy available from        Elgiloy

Specialty Metals Division of Elgin, Ill., U.S.A., which should be heatedto a recrystallization temperature ranging from 1090 to 1150° C.;

-   -   3. Platinum iridium (Pt Ir) alloys, which should be heated to a        recrystallization temperature ranging from 1000 to 1200° C. for        alloys having up to ten percent iridium, and ranging from 1300        to 1500° C. for alloys having greater than ten percent iridium;    -   4. Nickel-titanium (Ni Ti) alloys (e.g., nitinol having        stoichiometry around 50-50 for shape memory properties), which        should be heated to a recrystallization temperature ranging from        650 to 950° C., with longer hold times for the lower        temperatures;    -   5. Tantalum (Ta) and tantalum based alloys, such that pure        tantalum is heated to a recrystallization temperature ranging        from 1260 to 1370° C., with temperatures for tantalum alloys        depending on the particular alloy; and    -   6. Titanium (Ti) and titanium based alloys, such that pure        titanium is heated to a recrystallization temperature ranging        from 650 to 750° C., with temperatures for titanium alloys        depending on the particular alloy.

Example 1

A 316L stainless steel specimen in the form of a rectangular bar,15×15×100 mm in size, is placed into a forging machine, such as the oneheretofore described at the Japanese NRIM. Air is then evacuated fromthe forging machine, and the stainless steel specimen is heated to 800°C. at a rate of about ten ° C./sec. The specimen is held in the machineat 800° C. for one minute to stabilize the specimen temperature, andthen forged immediately. The specimen is subjected cross-forging to anet deformation of eighty percent. The specimen is then held at 800° C.for up to five minutes to control the grain size to the desired level ofabout five microns. The specimen may then be air cooled or waterquenched to room temperature to halt the recrystallization process. FIG.2 depicts a transmission electron micrograph of a typical section of a316L stainless steel specimen resulting from this process.

Example 2

A 316L stainless steel specimen in the form of a rectangular bar,15×15×100 mm in size, is placed in a forging machine, such as the oneheretofore described at the Japanese NRIM. Air is then evacuated fromthe forging machine. The stainless steel specimen is then heated to 800°C. at a rate of about ten ° C./sec. The specimen is held for in themachine at 800° C. for one minute to stabilize the specimen temperature,and then forged immediately. The specimen is then subjected tocross-forging to a net deformation of eighty percent. The specimen isthen cooled to room temperature in water. The specimen is subsequentlyheated to 900° C. at a rate of about ten ° C./sec and held at thattemperature for two minutes, and then air cooled or water quenched toroom temperature to halt recrystallization. FIGS. 3A and 3B depicttransmission electron micrographs of typical sections of a 316Lstainless steel specimen resulting from this process.

Stents are well known in the art and can have many different types ofpatterns and configurations. The following description of intravascularstents, as shown in FIGS. 4-11, are typical stent patterns made fromstainless steel tubing. Other patterns are well known in the art, andthe description herein of stents and delivery systems is by way ofexample and is not meant to be limiting.

Referring to FIG. 4, a stent 60 constructed from a fine grain material,such as 316L stainless steel, may be mounted on a catheter assembly 62,which is used to deliver the stent and implant it in a body lumen, suchas a coronary artery, peripheral artery, or other vessel or lumen withinthe body. The catheter assembly includes a catheter shaft 63, which hasa proximal end 64 and a distal end 66. The catheter assembly isconfigured to advance through the patient's vascular system by advancingover a guide wire 68 by any of the well known methods utilizing an overthe wire system (not shown), or a rapid exchange (RX) catheter system,such as the one shown in FIG. 4. The guide wire may also be constructedfrom a fine grain material according to the processes of the presentinvention

Catheter assembly 62, as depicted in FIG. 4, is of the well known rapidexchange type that includes an RX port 70, where the guide wire 68 willexit the catheter shaft 63. The distal end of the guide wire exits thecatheter distal end 66 so that the catheter advances along the guidewire on a section of the catheter between the RX port and the catheterdistal end. As is known in the art, a guide wire lumen (not shown) isconfigured and sized for receiving various diameter guide wires to suita particular application.

The fine grain stent 60 is typically mounted on an expand able member(balloon) 72 positioned proximate the catheter distal end 66. The stentis crimped tightly thereon, so that the stent and expandable memberpresent a low profile diameter for delivery through the patient'svasculature. The stent may be used to repair a diseased or damagedarterial wall 74, which may include plaque 76, a dissection or a flapthat are commonly found in the coronary arteries, peripheral arteriesand other vessels. Such plaque may be treated by an angioplasty or otherrepair procedure prior to stent implantation.

In a typical procedure to implant a stent 60 formed from a fine grainmaterial, the guide wire 68 is advanced through the patient's vascularsystem by well known methods so that the distal end of the guide wire isadvanced past the plaque or diseased area 76. Prior to implanting thestent, the cardiologist may wish to perform an angioplasty procedure orother procedure (e.g., atherectomy) in order to open the vessel andremodel the diseased area. Thereafter, the stent delivery catheterassembly 62 is advanced over the guide wire so that the stent ispositioned in the target area. During positioning and throughout theprocedure, the fine grain stent may be visualized through x rayfluoroscopy and/or magnetic resonance angiography.

As shown in FIG. 5, the expandable member or balloon 72 is inflated bywell known means so that it expands radially outwardly and in turnexpands the fine grain stent 60 radially outwardly until the stent isapposed to the vessel wall 74. The balloon is fully inflated with thestent expanded and pressed against the vessel wall. The expandablemember is then deflated, and the catheter assembly 62 is withdrawn fromthe patient's vascular system. The guide wire 68 typically is left inthe vessel for post dilatation procedures, if any, and subsequently iswithdrawn from the patient's vascular system. As depicted in FIG. 6, theimplanted stent remains in the vessel after the balloon has beendeflated and the catheter assembly and guide wire have been withdrawnfrom the patient.

The stent 60 formed from a fine grain material serves to hold open theartery wall 74 after the catheter assembly 62 is withdrawn, asillustrated by FIG. 6. Due to the formation of the stent from anelongated tubular member, the undulating components of the stent arerelatively flat in trans verse cross section, so that when the stent isexpanded, it is pressed into the wall of the artery and as a result doesnot interfere with the blood flow through the artery. The stent ispressed into the wall of the artery and will eventually be covered withendothelial cell growth, which further minimizes blood flowinterference. The undulating ring portion of the stent provides goodtacking characteristics to prevent stent movement within the artery.Furthermore, the closely spaced cylindrical elements at regularintervals provide uniform support for the wall of the artery, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of the artery, as illustrated in FIGS. 5 and6.

As shown in FIGS. 7-11, the fine grain stent 60 is made up of aplurality of cylindrical rings 80, which extend circumferentially aroundthe stent. The stent has a delivery diameter 82 (FIG. 7), and animplanted diameter 84 (FIG. 8). Each cylindrical ring 80 has a proximalend 86 and a distal end 88. When the stent is laser cut from a solidtube, there are no discreet parts, such as the described cylindricalrings. However, it is beneficial for identification and reference tovarious parts to refer to the cylindrical rings and the following partsof the stent.

Each cylindrical ring 80 defines a cylindrical plane 90, which is boundby the cylindrical ring proximal end 86, the cylindrical ring distal end88 and the circumferential extent as the cylindrical ring 80 traversesaround the cylinder. Each cylindrical ring includes a cylindrical outerwall surface 92, which defines the outer most surface of the fine grainstent 60, and a cylindrical inner wall surface 93, which defines theinnermost surface of the stent. The cylindrical plane follows thecylindrical outer wall surface.

As shown in FIGS. 9 and 10, the stent 60 may be constructed with struts100 formed from fine grain material having a variable thickness alongthe stent length. As one example, it is contemplated that struts 102 atthe ends of the stent may be thicker than the struts 104 in the centerof the stent for purposes for increased radiopacity and to counternon-uniform balloon expansion. When the balloon first inflates, theballoon ends have a tendency to inflate at a faster rate than theballoon center. However, with thicker struts at the stent ends, theballoon, and hence the stent, will expand more uniformly.

Referring to FIGS. 8, 10 and 11, each adjacent cylindrical ring 80 maybe connected by at least one undulating link 110 or straight link 112.The stent may include only straight links (FIG. 11), may include onlyundulating links (FIG. 8) or may include both undulating links andstraight links (FIG. 10) to connect adjacent cylindrical rings. Both thestraight links and the undulating links assist in preventing stentforeshortening. Further, the straight links may provide more stabilityand rigidity in a localized area, such as at the stent ends, such thatit may be desirable to incorporate more straight links between thecylindrical rings at the stent ends, than in the center of the stent. Anundulating link may be positioned substantially within the cylindricalplane 90, as defined by the cylindrical outer wall surface 92 and thecylindrical inner wall surface 93.

The fine grain stent 60 of the present invention can be made in manyways. One method of making the stent is to cut a thin-walled tube offine grain material to remove portions of the tubing in the desiredpattern for the stent, leaving relatively untouched the portions of themetallic tubing that are to form the stent. In accordance with theinvention, it is preferred to cut the tubing in the desired pattern bymeans of a machine-controlled laser, as is well known in the art. Othermethods of forming the stent of the present invention can be used, suchas chemical etching; electric discharge machining; laser cutting a flatsheet and rolling it into a cylinder with a longitudinal weld; and thelike, all of which are well known in the art at this time. In addition,the stent and/or its struts may be formed from a wire or elongated fiberconstructed from a fine grained material. The cross-section of suchstruts may be round, rectangular or any other suitable shape forconstructing a stent.

Referring now to FIGS. 12 and 13, and by way of example, the presentinvention may be incorporated into an embolic protection device 120.Such device may include a filter assembly 122 and expandable strutassembly 124. The embolic protection device may further include anelongated tubular member 130, within which may be disposed a guidewire132 for positioning the device within a corporeal lumen. In accordancewith the present invention, the embolic protection device may include aplurality of longitudinal struts 126 and transverse struts 128 that areconstructed from a fine grain material according to the presentinvention. In addition, the filter assembly may be formed from a finegrain material as heretofore described. Similarly, the guidewire mayinclude or be constructed from a fine grain material, and the distal endof the guidewire 134 may also include or be constructed from a finegrain material.

Referring now to FIG. 14, fine grain material of the present inventionmay be incorporated into a bifurcated graft 140. Likewise, the finegrain material may be incorporated into a tubular graft (not shown).Such a graft includes a DACRON, TEFLON or other suitable flexiblematerial having an upper body 142, a first leg 143 and a second leg 144,wherein the legs are joined to the upper body. Such a configurationforms a “Y” or “pants leg” configuration. A plurality of closely spacedmarkers 146 formed from a radiopaque fine grain material may beconfigured on the outside of the first and second legs. Similarly, widerspaced markers 148 may be configured on the inside of the legs of thebifurcated graft (or visa versa). Such markers may be formed fromradiopaque fine grain material as heretofore described, which may besewn, glued or otherwise bonded to the graft.

In many such grafts 140, such as those used for repairing an abdominalaortic aneurysm, the upper body may include a first attachment system150 positioned proximate to an upper opening of the graft. Tube graftsmay contain a like attachment system at the lower opening of the graft.Similarly, bifurcated grafts may include smaller attachment systems 152positioned at the end of the legs and proximate to the lower openings ofthe graft. As heretofore described regarding stents (FIGS. 4-11), theattachment system may be made of a variety of fine grain materials inaccordance with the present invention. Such stents and attachmentsystems may be of various configurations, such as, but not limited to, aring and link design, a zigzag design, a coil design or a tubular meshdesign.

While particular forms of the invention have been illustrated anddescribed with regard to certain medical devices, it will also beapparent to those skilled in the art that various modifications can bemade without departing from the scope of the invention. Morespecifically, it should be clear that the present invention is notlimited to catheters, tubular type stents, embolic protection devicesand endovascular grafts. Likewise, the invention is not limited to anyparticular method of forming the under lying medical device structure.While certain aspects of the invention have been illustrated anddescribed herein in terms of its use as an intravascular stent, it willbe apparent to those skilled in the art that the stent can be used inother body lumens. Further, particular sizes and dimensions, materialsused, and the like have been described herein and are provided asexamples only. Other modifications and improvements may be made withoutdeparting from the scope of the invention. Accordingly, it is notintended that the invention be limited, except as by the appendedclaims.

1-40. (canceled)
 41. An intravascular prosthesis for use in a bodylumen, comprising: a plurality of structural members interconnected toform the intravascular prosthesis; and each structural member comprisingat least one material selected from the group consisting of a Co—Cralloy, a Pt—Ir alloy, a Ni—Ti alloy, tantalum, and a tantalum-basedalloy, each structural member having a thickness of 100 μm or less andincluding at least ten grains spanning the thickness, the at least tengrains spanning the thickness of each structural member to allowdistribution of stresses within some grains of the at least ten grainsto be distributed to other grains of the at least ten grains.
 42. Theintravascular prosthesis of claim 41, wherein the each of the pluralityof structural members has at least 15 grains spanning the thickness. 43.The intravascular prosthesis of claim 41, wherein the each of theplurality of structural members has at least 20 grains spanning thethickness.
 44. The intravascular prosthesis of claim 41, furthercomprising at least one straight link attaching a structural member toan adjacent structural member.
 45. The intravascular prosthesis of claim41, further comprising at least one undulating link attaching astructural member to an adjacent structural member.
 46. Theintravascular prosthesis of claim 41, further comprising at least oneundulating link attaching a structural member to an adjacent structuralmember, and at least one straight link attaching a third a structuralmember to a fourth structural member.
 47. The intravascular prosthesisof claim 41, wherein the intravascular prosthesis is a stent.
 48. Anintravascular stent for use in a body lumen, comprising: a plurality ofcylindrical rings interconnected to form the stent, each cylindricalring having a first delivery diameter and a second expanded diameter;and each cylindrical ring comprising at least one material selected fromthe group consisting of a Co—Cr alloy, a Pt—Ir alloy, a Ni—Ti alloy,tantalum, and a tantalum-based alloy, each cylindrical ring having athickness of 100 μm or less and including at least ten grains spanningthe thickness, the at least ten grains spanning the thickness of eachcylindrical ring to allow distribution of stresses within some grains ofthe at least ten grains to be distributed to other grains of the atleast ten grains so as to increase strength and ductility of eachcylindrical ring.
 49. The intravascular stent of claim 48, wherein thematerial has at least 15 grains spanning the thickness.
 50. Theintravascular stent of claim 48, wherein the material has at least 20grains spanning the thickness.
 51. The intravascular stent of claim 48,further comprising at least one straight link attaching each cylindricalring to an adjacent cylindrical ring.
 52. The intravascular stent ofclaim 48, further comprising at least one undulating link attaching eachcylindrical ring to an adjacent cylindrical ring.
 53. The intravascularstent of claim 48, further comprising at least one undulating linkattaching a first cylindrical ring to a first adjacent cylindrical ring,and at least one straight link attaching a second cylindrical ring to asecond adjacent cylindrical ring.
 54. The intravascular stent of claim48, wherein each cylindrical ring includes a proximal end, a distal endand a cylindrical wall extending circumferentially between the proximalend and the distal end, and further including an undulating linkpositioned substantially within the cylindrical wall of a firstcylindrical ring so as to attach the first cylindrical ring to anadjacent cylindrical ring.
 55. An intravascular stent for use in a bodylumen, comprising: a plurality of struts interconnected to form a stent,the stent having a first delivery diameter and a second expandeddiameter; and each strut comprising at least one material selected fromthe group consisting of a Co—Cr alloy, a Pt—Ir alloy, a Ni—Ti alloy,tantalum, and a tantalum-based alloy, each strut having a thickness of100 μm or less and a recrystalized grain size with at least ten grainsspanning the thickness, the at least ten grains spanning the thicknessof each strut to allow distribution of stresses within some grains ofthe at least ten grains to be distributed to other grains of the atleast ten grains.
 56. The intravascular stent of claim 55, wherein theeach of the plurality of struts has at least 15 grains spanning thethickness.
 57. The intravascular stent of claim 55, wherein the each ofthe plurality of struts has at least 20 grains spanning the thickness.58. The intravascular stent of claim 55, wherein the at least onematerial is a tantalum-based alloy.
 59. The intravascular stent of claim55, wherein the at least one material is a Co—Cr alloy.
 60. Anintravascular prosthesis for use in a body lumen, comprising: aplurality of structural members interconnected to form the intravascularprosthesis; and each structural member comprising at least one materialselected from the group consisting of a cobalt alloy, a precious metalalloy, and a refractory metal alloy, each structural member having athickness of 100 μm or less and including at least ten grains spanningthe thickness, the at least ten grains spanning the thickness of eachstructural member to allow distribution of stresses within some grainsof the at least ten grains to be distributed to other grains of the atleast ten grains.
 61. The intravascular stent of claim 60, wherein theat least one material is a cobalt alloy.
 62. The intravascular stent ofclaim 60, wherein the at least one material is a precious metal alloy.63. The intravascular stent of claim 62, wherein the precious metalalloy comprises a platinum-iridium alloy.
 64. The intravascular stent ofclaim 60, wherein the at least one material is a refractory metal alloy.65. The intravascular stent of claim 64, wherein the refractory metalalloy comprises a tantalum-based alloy.